Field of the Invention
The invention relates to a bone anchoring device for anchoring a stabilization rod to a bone or to a vertebra. The bone anchoring device includes a bone anchoring element and a receiving part for receiving a head of the bone anchoring element and for receiving a stabilization rod to be connected to the bone anchoring element. The bone anchoring element is pivotably connected to the receiving part and can be fixed at an angle by exerting pressure onto the head via a pressure element which may be arranged in the receiving part. The head has a recess which accommodates a closed ring which is configured to cooperate in such a way with the head and the pressure element that the pressure element can assume a position within the receiving part in which it clamps the head via the ring by friction without fully locking the head with respect to the receiving part.
Description of Related Art
US 2004/0267264 A1 describes a polyaxial fixation device, wherein the polyaxial bone screw includes an engagement member that is adapted to provide sufficient friction between the spherical head and the receiver member to enable the shank to be maintained in a desired angular orientation before locking the spherical head within the receiver member. The engagement member is realized, for example, by an open snap ring around the head or by spring members provided at the compression cap to frictionally engage the spherical head or by a slot provided in the compression cap.